1. Field of the Invention
The invention relates to a process and apparatus for making optical fibers from core and cladding glass rods and to the fibers made by the process. More particularly, the invention relates to separately melting core and cladding glass rods and combining the melts proximate a fiber drawing orifice so that the core glass is surrounded by the cladding glass and drawing a glass clad optical fiber from the combined melts.
2. Description of the Background Art
Optical fibers, windows and filters find increasing use for many applications, particularly in data transmission. For example, silica based optical fibers are widely used in the telecommunications industry. However, silica fibers transmit only up to about 2 microns and there are many applications in which the wavelengths are longer than 2 microns, such as infrared imaging, detection and analysis of high temperatures and high temperature effects and power delivery from CO and CO.sub.2 lasers. Remote fiber optic chemical sensing systems are useful for the clean up of Department of Defense and Department of Energy facilities, as well as other industrial applications, because practically all molecular species possess characteristic vibrational bands in the infrared region between 3-11 microns. Zirconium fluoride based fibers transmit to about 3.5 microns, but this still isn't sufficient for most infrared systems. Chalcogenide glasses transmit to beyond 10 microns and are therefore used for optical fibers in fiber optic based sensor systems using evanescent, absorption and diffuse reflectance spectroscopies, which require long wavelength infrared transmission capability. Since the efficiency and capability of such systems depends in large measure on the infrared optical properties of the glass, it is important that the glass have low transmission losses. Therefore, there is a need to fabricate low loss chalcogenide glass fibers and especially in long lengths, to enhance the capabilities of many systems. For practical applications the chalcogenide glass fibers need to be glass clad to eliminate unwanted evanescent absorption and bending losses. Core and cladding glass compositions are selected so that the core refractive index is higher than that of the cladding while maintaining similarity in thermal properties. Typical techniques used to fabricate glass clad chalcogenide glass fibers include drawing the clad fiber from a preform fabricated by collapsing a cladding glass tube onto a core glass rod within. However, significant transmission losses can and do occur with the use of glass clad chalcogenide fibers drawn from such preforms due to bubbles in both the core and cladding glass and at the core/cladding glass interface, and also due to soot particles at the core/cladding glass interface caused by fabrication of the preforms and drawing of the clad fibers. These bubbles and soot particles act to scatter the infrared signals being transmitted which results in significant transmission losses. Further, practical size limitations of the preforms limit the process to drawing multimode fibers and the lengths of fiber drawn to typically less than 100 meters. U.S. Pat. No. 4,908,053 discloses drawing a clad fiber from a composite of a glass core rod concentrically disposed within a cladding glass tube in which a space exists between the tube and rod by melting the composite only at the bottom of the crucible in the vicinity of the drawing nozzle. The melting collapses the tube onto the rod only in the melt zone and the composite slowly moves down through the furnace as it is used up. While this process avoids the use of a core/clad preform, it does not prevent bubbles or soot formation at the core/cladding glass interface.
In order to avoid the need for preforms, double crucible processes have been developed in which a core glass crucible is concentrically disposed inside a cladding glass crucible so that the cladding glass melt is in contact with the outside of the core glass crucible. Both crucibles have a hole or orifice concentrically placed in the bottom of the crucible for the glass melts to flow out of, with both orifices coaxial and with the orifice in the bottom of the core glass crucible disposed just above the orifice in the cladding glass crucible. As the core glass melt flows out the orifice through the bottom of the core glass crucible, it contacts and is surrounded by the cladding glass melt and both melts flow out of the orifice in the bottom of the cladding glass crucible and form a clad fiber which is called a core/clad fiber. One such process is disclosed, for example, in U.S. Pat. No. 4,897,100 in which core and cladding glass chunks are melted in two separate, but concentric crucibles, with the core glass crucible disposed inside the cladding glass crucible. Each crucible has an orifice at the bottom for drawing out the molten glass, with the core glass crucible orifice disposed just above the cladding glass crucible orifice. Both orifices are coaxial. As the core glass melt flows out the orifice in the bottom of the core glass crucible, it is surrounded by cladding glass flowing down through the orifice in the bottom of the cladding glass crucible and a core/clad fiber is drawn. In this process, melting the glass chunks in the crucibles introduces gas bubbles at the interfaces and interstices of the chards or chunks as they melt. As a consequence, the glass melts are held at elevated temperatures for long periods of time to drive out some of the gas and to achieve homogeneity of the melt. Unfortunately, this can change the composition of the glass over a period of time as more volatile components of the glass are vaporized. Both glass melts are simultaneously withdrawn from the orifice at the bottom of their respective crucibles, with the core glass melt flowing through the cladding glass melt below, so that the cladding glass flows around the core glass as both glasses flow out the bottom of the cladding glass crucible. This process is difficult to control, uniform concentricity of the core and cladding glasses is extremely difficult to achieve, and it does not eliminate bubbles or soot formation. Another approach to the double crucible process is one in which a core glass disk and a cladding glass disk are core drilled from large slabs of glass. The core glass disk is heated and melted in a crucible having a hole in the bottom from which is drawn a core glass fiber. The cladding glass disk is heated in a separate crucible coaxial with and disposed vertically below the core glass crucible and it also has a hole or orifice in the bottom. The solid glass fiber drawn from the core glass crucible passes through the cladding glass melt which coats the core fiber with cladding glass and a glass clad fiber is drawn out the bottom of the cladding glass crucible. Since the solid core glass fiber must pass through the cladding glass melt, both glasses must have a different viscosity profile and the core glass must have a higher melting temperature. Aside from inherent stress, bubbles and soot are formed at the core and cladding glass interface of the fiber produced from this process. Also, the clad fiber has a low melting temperature and cannot generally be used above 110.degree. C., which means that it cannot be used for high power lasers. Still further, core drilling the core and cladding glass disks from large slabs of glass can introduce contaminants onto the glass. None of these double crucible processes is suitable for use with the relatively volatile and unstable chalcogenide glass compositions as both glass compositions remain in the molten state for a long period of time and the resulting volatilization losses lead to compositional variations in the core and cladding glasses, which itself leads to increased optical losses. Therefore, there is still a need for a method of producing core/clad glass optical fiber without the need for a core/clad preform or the use of glass chunks, with little or no soot formation at the interface between the glasses, and which will also eliminate or at least minimize the size and frequency of bubbles present in the glasses.